If you do enough planning, Den Haag, has a tram and bus system, so you could theoretically eliminate some of the urban walking. I don't see it as urgent for 8 and 5 to see the two (and more) excellent art museums there - although I love art museums myself. In particular the Gemeentmuseum is just immense.
Do you ever use public transportation at home? It's important to understand waiting around for public transit. And going to two places by transit in one day has a lot of waiting and sitting for people used to an SUV direct to the soccer field. Keukenhof is a special event, so transit to and from there is an addition. You can't make a continuous trip without retracing your steps. Like if you wanted to bus to a town north of Boston in the morning and a town south of Boston in the afternoon.
Note that Keukenhof is not just a few bulb beds. It's the equivalent of a medium-sized public garden anywhere else. And that's just settling for a view of the (separate) bulb fields visible downhill from the perimeter fence of Keukenhof. I'll guess that Keukenhof is as big as all of the formally planted areas at Tower Hill or Garden in the Woods (MA) but not including open meadows or forest walks in those places. There are also periodic entertainments, like a brass band or a barrel-organ concert, many dining options, and pop-up events around the Keukenhof property. Think Knott's Berry Farm but without the rides.
You know your family capabilities and interests, and because the season is so short, Keukenhof alone is a must for this trip. But Amsterdam is a very rich destination, including for young people. Taking a daytrip out of 3 days is a big loss of less exhausting visits within resting distance of your hotel in Amsterdam. Be careful about your planned pace for the whole trip.